1. Field of the Invention
The present invention relates to an arrangement for controlling a constant signal amplitude in an opto-electronic scanning system, for example in an incremental angle indicator, where diaphragms are periodically interposed between a light source and a light receiver and where the brightness of the light source can be regulated by a light sensitive receiver which scans the light source.
2. Description of the Prior Art
In addition to magnetic or capacitive scanning systems, optoelectronic scanning systems are widely used for determining the position and condition of elements, in particular of moving elements. Therefore, for example, pulse train discs which are arranged on drive shafts and which are alternately provided with openings and screened areas, are passed between a light source and a light receiver of the opto-electronic scanning system, as a result of which it is possible to derive signals which can be used to determine the position of the drive shaft. The light receivers, for example photo transistors or photo resistors, and the following light amplifier and analysis circuits operate particularly well under conditions characteristic of these circuits which leads to the recognition that the brightness of the light source can be designed to be at an optimum, in particular also with respect to the general surrounding brightness and to stray light produced. Accordingly, it is desirable to design the light source for an optimum brightness. Fluctuations in the supply voltage and other environmental influences change both the characteristics of the brightness of the light source and also the characteristics of the light receiver. Optimum characteristics can be reestablished by regulating the brightness of the light source.
This regulation of the brightness of the light source can be carried out automatically in that the light source is constantly scanned by a further light receiver and the received value is used to regulate the brightness of the light source. This measure involves an additional expense and requires that the surface of the light source be sufficiently large that an additional light receiver is covered in an equivalent fashion.
Using the light receiver of the opto-electronic scanning system itself, in order to regulate the brightness of the light source, presents difficulties for the reason that when the light receiver is darkened by diaphragms moved between the light receiver and the light source for the purpose of scanning, the light source would become overmodulated.